VARIATION IN THE DESMIDIER. 415 
much reduced in length, and a total suppression of them resulted 
in the genus Didymoprium, the latter genus ultimately producing 
Gymnozyga by an elongation of the cells. Ayalotheca dissiliens 
may probably have had an origin from Didymoprium quadratum, 
D. equale, or some allied form, and other species of Hyalotheca, 
such as H. neglecta, which so nearly resembles a species of 
Gymnozyga, may have had an origin from the latter genus. 
At present, however, we know very little about many of the 
filamentous Desmidie®, and for this reason any remarks as to 
their origin must necessarily be mostly conjectural. 
The accompanying table (p. 414) of the phylogeny of the genera 
of Desmids may serve to illustrate the foregoing hy pothesis. 
EXPLANATION OF THE PLATES. 
a, a', a! —cell or semicell from front view. 
06,0, WEN » » vertical view. 
c =, " » side view. 
d = basal view of semicell. 
PLATE 8. 
Figs. 1-12. Penium spirostriolatum, Barker. 1, specimen from Oughtershaw 
Tarn, W. Yorks., x520; 2, from Llyn Padarn, N. Wales, x 520; 
3, from Castletown, S.W. Ireland, x520; 4, from Minnesota, 
U.S.A., x520; 5, from Cromagloun, S.W. Ireland, x520; 6, one 
apex of fig. 5, x 830 ; 7, from Cromagloun, S. W. Ireland, x520; 
8-10, apices of three specimens from Oughtershaw Tarn, W. Yorks., 
x1280 ; 11-12, portions of cell-wall of a specimen from Llyn Padarn, 
N. Wales, K 1280. 
» 19-19. Euastrum Didelta, Ralfs. x220. 13, specimen from Thursley 
Common, Surrey; 14, from Blea Tarn, Westmoreland; 15 & 16, 
from near Clapham, W. Yorks. ; 17, from Widdale Beck, N. Yorks.; 
18 & 19, from Castletown, S.W. Ireland. 
» 20-22, Xanthidium Smithii, Arch., var. variabile, Nordst. x520. 20, 
specimens from Esher Common, Surrey; 21 & 22, from near 
Devil's Jumps, Frensham, Surrey. 
PLATE 9. 
Figs. 1-8. Micrasterias Thomasiana, Arch., and intermediate forms between 
this species and M. denticulata, Bréb. 1 & 2, specimens from 
Terrington, N, Yorks., X170; 38 & 4, from Wrynose, Lancashire, 
x170; 5, from Thursley Common, Surrey, X170; 6, from Singa- 
pore, X390; 7 & 8, from Glen Shee, Perthshire, Scotland, x800 
(8 d is slightly oblique). 
LINN, JOURN.— BOTANY, VOL. XXXIV. 2G 
